This invention relates in general to techniques for distance measurements and in particular to a rangefinder with range to frequency conversion and method for using the rangefinder.
Several techniques for measuring distance with light signals are in common use today. They can be divided in two general categories, those that use the speed of light in some way to determine distance, and those that do not. The latter group usually uses a light projector located some distance from both the surface being lit and the detector. The detector then measures the direction of the light from both the surface being lit and the detector and triangulation is used to determine the surface's position. The accuracy of these systems depends on the separation distance between the emitter and detector, and typically works over a relatively narrow range of distances.
The distance measurement device disclosed here uses the speed of light as the basis for its distance measurement. It does this in a way that has not been used before and that provides several advantages to current techniques, as will be shown. Many of the problems of the currently used techniques described below are solved, and absolute distance is measured in a way that is potentially more accurate than either of these methods. As will be evident, this technique also has the ability to accomplish ranging at a lower cost than these systems.
There are several common methods of distance measurement based on the speed of light. The simplest of these is to emit a pulse of light and measure the time it takes for the reflected light to return to the detector, which is generally located close to the light source. The resolution is limited by the ability of the timing system used to discriminate small differences in time, and by factors such as the ability to determine the exact center of a return signal pulse which typically varies in amplitude. This method is suitable for measuring long distances, such as the distance to the moon, since the pulses can be of high power and detectable at long range.
Another method of deriving distance from the speed of light is to transmit an amplitude modulated light beam and to compare the phase of the return signal with the phase of the outgoing signal. The relative phase of the two signals depends on the distance to the subject and the frequency of the modulation used. The applications for this method are similar to the applications for the invention disclosed here.
The accuracy of the phase comparison method is limited by the ability of the phase detector used to resolve phase and the amount of isolation that can be obtained between the incoming and outgoing signals. Higher frequencies improve the resolution but worsen the crosstalk problem. This technique has been demonstrated to attain resolutions of 0.1 millimeters in the paper entitled "Laser Range Finding Sensor Robotics," by Clergeot et al. from ROVISEC-6, 1986. This method of ranging has the additional problem that the range reading aliases at range intervals equal to half the wavelength of the modulation. For example, with 50 MHz modulation (6 meter wavelength), it is not possible to distinguish between actual distances of n, n+3, n+6, . . . meters, since all of them will result in the same detected phase difference. Multiple frequencies or some other technique must be used to resolve this ambiguity in many practical applications. U.S. Pat. No. 3,649,123 discloses a method of optical distance measurement that uses a phase-locked loop (PLL) to create an oscillation frequency dependent on the range to the target. The PLL approach requires phase detection electronics that are susceptible to the same sources of error as conventional phase measurement. The PLL electronics require a frequency synthesizer that is complex, expensive and susceptible to errors. The PLL approach requires significant time to lock on to the frequency corresponding to the distance to the target. This is a disadvantage when the target is moving rapidly or when the beam is scanned to measure the distances of different objects in the environment.
U.S. Pat. No. 5,006,721 describes a LIDAR scanning system that uses phase angle detection to measure the distance to objects, and moving mirrors to deflect the beam and collect reflected light. The device in that patent uses separate transmit and receive optics to reduce the crosstalk. The patent describes the resultant problem of parallax, a well as the problem of phase ambiguity, both of which limit the depth of field of that device.
None of the above described systems is entirely satisfactory. It is therefore desirable to provide a distance measurement system in which the above described difficulties encountered by the above described systems are alleviated.